BUILD: threads/plock: fix a build issue on Clang without optimization

BUILD: threads/plock: fix a build issue on Clang without optimization

[ plock commit 4c53fd3a0b2b1892817cebd0db012a52f4087850 ]

Pieter Baauw reported a build issue affecting haproxy after plock was
included. It happens that expressions of the form :

     if ((const) ? (expr1) : (expr2))
       do_something()

always produce code for both expr1 and expr2 on Clang when building
without optimization. The resulting asm code is even funny, basically
doing :

     mov reg, 1
     cmp reg, 1
     ...

This causes our sizeof() tests to fail to build because we purposely
dereference a fake function that reports the location and nature of the
inconsistency, but this fake function appears in the object code despite
all conditions being there to avoid it.

However the compiler is still smart enough to optimize away code doing

    if (const)
       do_something()

So we simply repeat the condition before do_something(), and the dummy
function is not referenced anymore unless really required.